1. Field
The present invention relates to an optical pickup apparatus and an optical recording/reproducing system employing the same.
2. Description of the Related Art
As the industry develops, the amount of data to be processed and recorded increases, and an optical recording medium and optical recording/reproducing system having higher recording density are required. According to such requirement, optical recording/reproducing systems, for example, the Blu-ray Disc (BD) system and HD-DVD system have been suggested. However, these systems need to be compatible with an existing system such as CD or DVD. In particular, since light having a wavelength of 405 nm which is significantly shorter than conventional system is used in the BD system, an objective lens having a higher numerical aperture (NA) is required. Therefore, in order for all CD/DVD/HD-DVD/BD systems to be compatible, an optical pickup apparatus of the optical recording/reproducing system includes two objective lenses. In other words, an objective lens for a CD/DVD/HD-DVD system and an additional objective lens for a BD system is included in the optical pickup apparatus.
The method of using two objective lenses in one optical pickup apparatus includes preparing an additional actuator for each objective lens, and mounting all two objective lenses in one actuator. In addition, when mounting all two objective lenses in one actuator, two objective lenses may be arranged in a radial direction or in a tangential direction of a corresponding optical disc. When two objective lenses are arranged in a tangential direction of the optical disc due to a various structural reasons, one of two objective lenses falls off from the center line of the optical disc, which intersects the center of the optical disc, to the tangential direction, that is, offset of the lens occurs.
For example, as shown in FIGS. 1A and 1B, it is assumed that a first objective lens 15 disposed in the left side of an actuator 10 is arranged on the center line of a disk D. In FIG. 1A, although the first objective lens 15 moves along a radial direction of the disk D and tracks T1, T2, and T3 at any position, angles with respect to each track are not changed. However, since a second objective lens 15′ disposed in the right side of the actuator 10 is not arranged on an axis that passes through the center of the disk D, as shown in FIG. 1B, directions of tracks T1, T2, and T3 are changed according to the position of the second objective lens 15′ determined based on a radial direction of the disk D. Consequently, with respect to optical system combined with the second objective lens 15′, wherein the second objective lens 15′ is disposed in the right side of the actuator 10, accurate tracking error signals cannot be obtained using a conventional Differential Push-Pull (DPP) method.
Therefore, a conventional method of forming two or more sub beams on the tracks or shifting phases of all or a part of the sub beams by providing a specially shaped diffraction grating or a hologram optical element (HOE) to the optical system having the objective lens which is offset from the central line of the disk is used so that theoretically only a direct current (DC) exists in a sub push-pull signal. In addition, in order to reduce an effect due to a change of a track direction in the inner/outer circumference of discs, a direction of the diffraction grating is adjusted based on the center track T2, and the rest of the configuration of the optical system is made as if there is no offset of the lens.
However, in the conventional method, while the diffraction grating is rotated, an optical axis of a light source and an axis of the diffraction grating do not coincide, and thus, a difference in the amount of light is generated between sub-beams separated from the diffraction grating. Subsequently, an alternating current (AC) is substantially generated in sub push-pull (SPP) signals. In addition, when an optical axis of a light source and an axis of the diffraction grating coincide with each other, the centers of the sub-beams do not coincide with the boundary of the diffraction grating, and thus, an AC is generated in the SPP signal for each of two sub-beams. Moreover, in the conventional optical system, light emitted from the light source and incident on a track of the disk is only considered to control a direction of diffraction grating. Consequently, for light reflected from the disk track and incident on the photodetector, an angle of incidence may still be inaccurate. In other words, the main beam and a number of sub-beams are controlled to be accurately arranged along the track at the center of the disk, however, angles between the main beam and the sub-beams reflected from the track and photodetectors used to detect such beams may still be off. Accordingly, an AC is generated in the SPP signal, and phases of the SPP signal and a Main Push-Pull (MPP) signal are changed according to the radial position of the objective lens disposed on the disk. As a result, a tracking error signal is changed according to a radius of the disk as shown in FIG. 2. Since the tracking error signal is changed according to a radius of the disk, accurate servo-control of the optical pickup apparatus is difficult.